It is often operationally and economically advantageous to obtain measurements of certain parameters of interest during the drilling of a well borehole. Systems for obtaining measurements relating to the drilling operation are commonly referred to as measurement-while-drilling or “MWD” systems. MWD systems typically yield measures of a plurality of borehole conditions, the orientation and path of the borehole assembly, and other drilling related parameters of interest. Systems for obtaining measurements of characteristics of formation material penetrated by the borehole are commonly referred to as logging-while-drilling or “LWD” systems. LWD systems typically yield measures of formation porosity, formation density, fluid saturation information, bedding information and the like.
Numerous types of telemetry systems are used to transfer data, while drilling, between a borehole assembly and surface equipment disposed at the surface of the earth. Mud pulse systems are known in the art. Basic principles of mud pulse telemetry systems are disclosed in U.S. Pat. No. 3,958,217 “Pilot Operated Mud-Pulse Valve” and U.S. Pat. No. 3,713,089 “Data Signaling Apparatus for Well Drilling Tools”, both of which are herein entered into this disclosure by reference. U.S. Pat. No. 3,309,656 “Logging-While-Drilling System” discloses a mud pulse siren system, and is herein entered into this disclosure by reference. Electromagnetic telemetry systems are also known in the art. Basic principles of electromagnetic telemetry are disclosed in U.S. Pat. No. 4,525,715 “Toroidal Coupled Telemetry Apparatus” and U.S. Pat. No. 4,302,757 “Borehole Telemetry Channel of Increased Capacity”, both of which are entered herein into this disclosure by reference. Within the context of this disclosure, the term “drilling system” includes both MWD and LWD systems.
Telemetry data transmission rates or telemetry bandwidths of LWD or MWD systems are relatively small in relation to comparable wireline systems. Although sensors disposed in borehole drilling assemblies may be as sophisticated as their wireline counterparts, real time measurements recorded at the surface of the earth are typically limited by LWD and MWD telemetry bandwidths. Redundant or parallel telemetry from a given sensor can increase telemetry bandwidth.
LWD and MWD telemetry systems are often “noisy” resulting from harsh conditions encountered in a borehole drilling environment. Again, redundant telemetry from a given sensor can optimize the flow of valid data between the sensor within the borehole assembly and the surface of the earth.
It is often desirable to make LWD and MWD measurements simultaneously while drilling. As an example, measurement of a formation parameter, such as formation resistivity, can be used as a criterion for controlling the direction in which the drill bit advances the borehole. This methodology is commonly referred to as “geosteering”. The geosteering methodology requires simultaneous transmission of real-time MWD data from both a rotary steerable device and transmission of real-time data from at least one LWD sensor. The physical layout of a typical borehole assembly portion of a drilling system can introduce problems in telemetering both LWD and MWD data using a single telemetry system. As an example, a mud motor may segregate and electrically isolate the rotary steerable device and related sensors from a borehole assembly subsection comprising LWD sensors. Typically the rotary steerable device is disposed below the mud motor and the LWD sensor subsection is disposed above the mud motor. Any type of electrical connection through the mud motor is typically unreliable or logistically impractical. As a result, simultaneously transmit of both MWD and LWD data using this methodology with a single telemetry system is also typically unreliable or logistically impractical. Limited range or “short-hop” electromagnetic or acoustic transmission systems have been used to telemeter LWD data uphole past a mud motor to a single downhole telemetry unit for subsequent transmission to the surface. These systems typically have relatively narrow bandwidths, are unreliable in certain types of borehole environs, and add fabrication and maintenance costs to the borehole measure system.